Study On Microstructure And Properties Of Hot Stamping Steel Produced By CSP Process

Compared with the conventional hot rolling and cold rolling approach,using CSP(compact strip production line)short process to produce hot stamping steel can eliminate cold rolling link,simplify the production process and reduce production costs.However,the mechanism of physical metallurgy process in the rolling process of hot stamping steel needs proving,while the inhomogeneous of product organization and quenching process also need further studying.In this paper,the physical metallurgy simulation and quenching process optimization of 22MnB5 hot stamping steel produced by CSP process are studied,which provides a theoretical basis for the production of hot stamping steel and quenching quenching process optimization by CSP process.The evolution of austenitic tissue under CSP process conditions was investigated.The thermal deformation model of high temperature austenite region,the critical deformation degree model of dynamic recrystallization and the flow stress-strain model were established.The results exhibit that in order to ensure the uniformity of product organization,the slab should be deformed by more than 50 % at the first two rolling pass.The research results can provide a theoretical foundation for the setting of rolling system in actual production process.The continuous cooling transformation of 22MnB5 hot forming steel was studied,and its static and dynamic continuous cooling transformation diagram were obtained.The results show that the coiling temperature should be controlled more than 640℃,and the cooling rate after coiling is controlled at 2-3℃/s in the production process of hot stamping steel.In the process of hot stamping,in order to ensure the good performance of the forming parts,the cooling rate after forming should be greater than 25℃/s.The effects of cooling path on the microstructure and hardness of hot stamping steel after quenching were studied.The results manifest that setting the cooling speed above the starting temperature of martensite transformation can slow down the precipitation of carbide in tempering martensite and improve martensite hardness.To obtain higher hardness of the tissue,the cooling speed before martensite transformation should reaches 50℃/s,and the subsequent cooling speed should be higher than 8℃/s.The microstructures and mechanical properties of hot stamping steel sheet produced by CSP process were analyzed.The results indicate that the main microstructures of hot stamping steel produced by CSP process are ferrite and pearlite,the yield strength is 400-600 MPa,and the tensile strength is 580-760 MPa.The microstructure and mechanical properties of quenched hot stamping steel under the condition of 850-1000℃ and holding for 1-15 min were studied.The results show that the appropriate addition of Nb and Ti can refine the original austenite grain,thus significantly improve the strength of quenching martensite.Controlling the temperature at 900℃ and holding for 5 minutes can obtain the best comprehensive mechanical properties.